Dispersions of polymers used in printing inks, paints and toners are known in the art (for example, see U.S. Pat. Nos. 6,638,995 and 5,331,025, U.S. Patent Pub. No. US 2005/0244736 and International Patent Pub. Nos. WO 2009/155201, WO 2011/014826 and WO 2012/068153). International Patent Pub. No. WO 2009/155201 describes the use of a non-aqueous dispersion (NAD) polymer to increase the drying speed of a sheetfed offset ink. The specification mentions that the amount of NAD present in the sheet fed offset ink is at least from about 1% to about 15%, more preferably about 5% and most preferably about 10%. The examples, however, are limited to an amount of NAD of 10 wt % based on the weight of the ink, which necessitates the use of a metal drier or metal catalyst to polymerize the ink.
International Patent Pub. No. WO 2012/068153 describes the use of a non-aqueous dispersion (NAD) polymer in printing inks in order to increase the setting and rub resistance properties of the inks when applied to a substrate. The specification mentions that the amount of NAD present in the ink ranges from 11% to 60%, or 12% to 50%, and preferably 25% to 35% based on the weight of the ink composition.
Water-based dispersed (emulsion) polymers for use in water-based overprint varnishes are also known in the art. For example, see U.S. Pat. Nos. 4,916,171 and 7,071,261. U.S. Pat. No. 4,916,171 describes the use of aqueous core-shell polymers made up of an alkali-soluble polymeric shell and an alkali-insoluble polymeric core that can be used in water-based paints, inks and overprint varnishes. U.S. Pat. No. 7,071,261 discloses aqueous compositions, such as aqueous coating compositions, that contain aqueous dispersions of polymeric nanoparticles.
U.S. Pat. No. 4,469,826 describes the use of aqueous thermoplastic polymer dispersions in non-aqueous lithographic inks and ink vehicles. The water-based polymer dispersion is separated from the water and then the finely divided particles are re-dispersed into the ink, which can be a non-aqueous (e.g., oil-based) ink. The particles of the water-based dispersion do not naturally mix into the ink at the particle size at which they were created during polymer synthesis and instead must be transferred from the aqueous phase of a polymer latex into the water-immiscible, hydrophobic organic vehicle.
Core/shell polymer technology is well known in the polymer arts. For example, see U.S. Pat. Nos. 3,985,703; 4,264,678; 4,375,532; 4,419,471; 4,888,395; 4,916,171; 4,994,524; 5,006,592; 5,087,663; 5,186,993; 5,324,780; 6,203,973; 6,309,739; 6,605,404 and 7,939,103; and Kazemi et al. (2008) Small 4(10):1756-1762. Any of the core/shell polymers known in the art that are compatible with the varnish components can be used in the present invention.
There is a need for an oil-based overprint varnish that can be used to coat a substrate containing a printed image that will exhibit desirable properties, such as high gloss, resistance to yellowing, and rub resistance shortly after printing, while also minimizing the time required before further processing of the coated substrate. Water-based polymer dispersions have been used in water-based overprint varnishes to improve the properties of a coated substrate. Water-based polymer dispersions have been used in oil-based overprint varnishes to improve the properties of a coated substrate. However, a need exists for oil-based overprint varnishes containing non-aqueous polymer dispersions, particularly where the particles of the polymer dispersion naturally mix into the coating at the particle size which they were created during polymer synthesis in a non-aqueous medium and that exists as a stable dispersion in mineral oil. A need also exists for a coating that can be used directly on a substrate after printing that improves the properties of the printed substrate, such as high gloss, resistance to yellowing, and rub resistance shortly after printing.